1. Field of the Invention
The present invention relates to an X-ray generator apparatus having an X-ray tube which generates X-rays when applied with a high voltage obtained by increasing an input A.C. voltage by means of a step-up transformer or the like and rectifying the increased voltage.
2. Description of the Related Art
An example of this type of conventional X-ray generator apparatus is shown in FIG. 1. In order to enhance the performance and make the device small and lightweight, a frequency converter 2 for converting the frequency of a voltage supplied from an input power source (A.C. power source) is connected to the primary side of a high voltage transformer 3. An output voltage of the frequency converter 2 is increased by the high voltage transformer 3 and an output voltage of the high voltage transformer 3 is rectified by a high voltage rectifier 4. A rectified output of the high voltage rectifier 4 is applied between the anode and cathode of an X-ray tube 5 serving as an X-ray source.
The frequency converter 2 is generally formed of a rectifier for converting an input A.C. voltage to a D.C. voltage, a capacitor for filtering the D.C. voltage, and an inverter for converting the D.C. voltage from the capacitor to an A.C. voltage of a desired frequency. The frequency converter 2 converts the frequency fo (which is a commercial frequency and is generally 50/60 Hz) of the input A.C. voltage to a frequency f1 which is higher than the frequency fo and then applies the voltage to the high voltage transformer 3. As the output frequency f1 of the frequency converter 2 is set to be higher, the size and weight of the frequency converter 2 and high voltage transformer 3 can be reduced. Since the impedances of coils and capacitors generally vary according to the frequency, the capacitance and inductance can be reduced as the frequency is set higher if the impedances are kept unchanged. Since the capacitance and inductance vary in proportion to the size of the capacitor and coil, the size and weight of the frequency converter 2 and high voltage transformer 3 using the coil and capacitor can be reduced as the frequency becomes higher.
However, in the above X-ray generator apparatus, the output frequency f1 of the frequency converter 2 cannot be increased limitlessly and the upper limit thereof is determined by the characteristic of the high voltage transformer 3 for the following reason. FIG. 2 shows an equivalent circuit of the device shown in FIG. 1 in view of the secondary portion of the transformer 3. In FIG. 2, L1, L2 and M respectively denote the primary inductance, secondary inductance and mutual inductance of the high voltage transformer 3. N denotes the turn ratio (the number of turns of the secondary windings/the number of turns of the primary windings) of the transformer 3. In this case, in order to obtain a high output voltage, the high voltage transformer 3 is so designed that the number of turns of the secondary winding is set to be very larger than that of the primary winding, and thus the secondary inductance L2 is very larger than the primary inductance L1 and mutual inductance M. Therefore, the inductance of the secondary portion of the high voltage transformer 3 which is actually equal to (L2-M) as shown in FIG. 2 can be regarded as being equal to the secondary inductance L2 by neglecting M, and in the following explanation, it is assumed that the inductance of the secondary portion is equal to L2. Further, assuming that the equivalent impedance of a the X-ray tube 5 is Rx and the terminal voltage of the X-ray tube 5 is Ex and the rectifier 4 is omitted from being consideration since it does not relate to the terminal voltage Ex, then the secondary inductance L2 is serially connected to the impedance Rx. If the output frequency of the frequency converter 2 is f1, an impedance Z2 due to the secondary impedance L2 can be expressed by the following equation and it is understood that it varies in proportion to the output frequency f1 of the frequency converter 2: EQU Z2=2.pi..multidot.f1.multidot.L2 (1)
Further, the voltage Ex applied to the X-ray rube 5 is expressed as follows: EQU Ex=E2.multidot.Rx/(Rx+Z2) (2)
Since the turn ratio N is very large and thus the inductance (L1-M)/N.sup.2 can be neglected, a terminal voltage E2 of the mutual inductance M is expressed as follows using an output voltage E1 of the frequency converter 2: EQU E2=E1.multidot.N (3)
As is clearly understood from the equations (1) and (2), the impedance Z2 becomes higher as the output frequency f1 of the frequency converter 2 becomes higher, causing a problem that the voltage Ex applied to the X-ray tube 5 is lowered. For this reason, the output frequency f1 of the conventional frequency converter 2 has an upper limit of approximately 10 KHz and a higher frequency exceeding the upper limit cannot be attained. If the frequency is set to approximately 10 KHz, it is difficult to greatly reduce the size and weight of the transformer and rectifier circuit and noise may be generated from the transformer 3. The reason why the output frequency f1 of the frequency converter 2 can be increased only to approximately 10 KHz at most is that the secondary inductance L2 of the high voltage transformer 3 is very large.
In order to solve the above problem, it has been proposed to modify the primary portion of the high voltage transformer 3 as shown in FIGS. 3 and 4. In the circuit of FIG. 3, a capacitor C1 is serially connected to the primary winding of the high voltage transformer 3 to attain a series resonance operation on the primary portion. In the circuit of FIG. 4, a capacitor C2 is connected in parallel with the primary winding of the high voltage transformer 3 to attain a parallel resonance operation on the primary portion. However, in either circuit, a voltage on the primary portion of the high voltage transformer 3 is equivalently increased by the series resonance or parallel resonance operation. The inductance L1 of the primary portion is originally small and the resonance voltage is low, and therefore, in order to obtain the same voltage applied to X-ray tube 5 as that obtained in a case wherein no resonance circuit is connected, it is only possible to increase the output frequency of the frequency converter 2 to two or three times the output frequency set in a case wherein no resonance circuit is connected.
Further, in U.S. Pat. No. 4,545,005 (Mudde), the secondary winding of the high voltage transformer is divided into a plurality of sub-windings to increase the frequency of the high voltage transformer, the sub-windings are connected to rectifier circuits are serially coupled and applied to an X-ray tube. However, the high voltage transformer is not divided and the high voltage transformer can be regarded as being a single transformer, and an output of one frequency converter is simply connected to a single high voltage transformer. Therefore, like the conventional case shown in FIG. 1, it is only possible to increase the frequency to approximately 10 KHz at most.
Further, in U.S. Pat. No. 4,317,039 (Romandi), plural frequency converters and plural high voltage transformers are used, but in this conventional case, the object thereof is to reduce ripples and the object is attained by setting the phases of the plural frequency converters different from one another. Therefore, this reference does not aim to increase the frequency of the transformer and discloses that the frequency lies in the medium frequency range and amounts to approximately six to seven KHz.